1. Field of the Invention
Poly-p-phenylene terephthalamide fibers, long known for their light weight, high strength, and high modulus, have found wide acceptance in a great number of applications requiring their unique combination of properties. The wide acceptance has, however, given rise to a demand and need for fibers having still higher strength and modulus for use in still more demanding applications. Fibers having decreased solubility and chemical reactivity and increased overall crystallinity and resistance to moisture regain have been sought and are in demand.
2. Description of the Prior Art
U.S. Pat. No. 3,869,430, issued Mar. 4, 1975 on the application of H. Blades, discloses fibers of poly-p-phenylene terephthalamide and processes for making the polymer and the fibers. That patent is particularly concerned with a process for heat treating such fibers after the fibers have been dried. That patent discloses, generally, that fibers could be heat treated whether wet or dry; but, in the examples, teaches heat treatment only of dried fibers and, elsewhere in the specification, cautions against heat treating fibers at excessive heat for excessive time with the warning that decreased tenacity and decreased polymer inherent viscosity will result.
Japanese Patent Publications Nos. 55-11763 and 55-11764 published Mar. 27, 1980, disclose fibers of poly-p-phenylene terephthalamide having high modulus and high tenacity but with polymer exhibiting only moderate inherent viscosity. The processes of those publications are particularly concerned with a fiber-drawing step performed after coagulating the spun polymer and before drying the fibers. In the drawing step, the fibers are actually stretched to 20 to 80 or 90% of the maximum stretch attainable before break. After the stretching, the fibers are dried at various times and at temperatures above about 300 degrees and as high as 600 degrees for three seconds. The inherent viscosity of the polymer of fibers so-made is always disclosed to be less than the inherent viscosity of the starting polymer and there is no suggestion that the inherent viscosity might be increased by any heat treatment.
The Journal of East China Institute of Textile Science and Technology, Vol. 10, No. 2 (1984), pp. 30-34, discloses heat treatment of fibers under very slight tension. There is teaching that the treatment causes decomposition, branching, and cross-association with accompanying increases in molecular weight. Neither fiber modulus nor degree of crystallinity is mentioned.